The present invention relates to a unit for heat sealing plastic film material as used in machines for wrapping palletized stacks of commodities. There are currently numerous machines of the type in question available for purchase, consisting generally of a load-bearing frame of gantry type construction, and a beam of upturned `L` profile suspended rotatably from the center of the frame by the horizontal member of the `L` and set in motion by a drive system. The vertical member of the `L` carries a station serving to support a roll of the plastic wrapping film; as a rule this same station is also capable of a controlled sliding movement on the vertical member of the beam so that the stacked commodities can be enveloped to their full height. The palletized stacks of commodities for wrapping are positioned at the center of the machine, or rather the center of the gantry, either by a pallet lift truck (in the case of more simple equipment), or by means of a permanently installed horizontal conveyor running at floor level directly beneath the gantry; once the pallet is at a standstill in the required position, a restraint device is activated in such a way as to descend and stabilize the entire pallet load by engaging the top face of the stack of commodities. Thereupon, the free end of the film is secured to the pallet base or held fast by means of a gripper mechanism, and the drive system is activated: the upturned `L` beam begins circling the pallet, and the stacked commodities are enveloped gradually by the uncoiling film. Where the width of the film is less than the height of the stacked commodities (as will normally be the case), the roll of film naturally must be made to ascend and descend on the vertical member of the beam in order to ensure that the commodities are enveloped entirely from top to bottom after a given number of overlapping turns have been effected to obtain the required compaction.
In an improved machine disclosed in European Patent application n. 92830107.6 (by the same applicant) and illustrated in the accompanying FIG. 1 for ease of description, the horizontal member R of the `L` beam T is again rotatable about a vertical axis V in such a way as to allow of wrapping commodities P stacked on the pallet Pa, but positioned beneath the roll B of plastic wrapping film; more exactly, the horizontal member R rotates in a plane between a supporting frame I and a platform A on which to stand the pallet Pa, in such a way as to establish a center of rotation coinciding low in the machine with the vertical axis V, about which the beam T is driven in a first direction denoted S1 in FIG. 2. The machine also comprises means M by which the platform A, and with it the entire pallet load, is set in rotation about the same vertical axis V in the opposite direction S2 to that of the beam. In most instances the plastic film is drawn taut during the wrapping operation (by the application of a braking action to the roll), in order both to minimize the outlay on materials, and to increase the tension of the wrapping with the end in view of obtaining a greater tightness, especially where the commodities are stacked high.
All of the machines referred to above are equipped with units by which the tail end of the film is cut and secured to the pallet, or heat-sealed to the wrapping layer beneath. In the case of a heat-seal (the method to which the disclosure specifically relates), such units comprise a support encased by a housing and designed to emit a reaction element positioned substantially to the side of the space occupied by the rotating pallet, placed along the path of the film and serving as a striker for the heat-seal plate, which is followed in the direction of rotation by the cutter.
Conventionally, the height of the heat-seal unit is kept within rational limits by reducing the width of the wrapping film along a portion of the tail end immediately before the sealing operation takes place, through the agency of a restraining element embodied typically as a hook capable of movement in the vertical direction; with the horizontal member of the `L` frame at standstill, the top edge of the film is intercepted by the hook and drawn downwards in such a way as to gather the deformable film and thus reduce its width, whereupon heat is applied to seal the gathered portion and secure the wrapping over the pallet.
Such a unit betrays certain drawbacks nonetheless, attributable in part to the limited space afforded by the plane of operation in which the various elements mentioned above are mounted and required to function; this can indeed be a critical factor in allowing the smooth and precise operation of the solution illustrated in FIG. 1, where there are two planes in contrarotation. A further negative factor is discernible in the use of the moving hook type element, which tends always to be imprecise when engaging the film, being liable either to penetrate the wrapping layers already in place or to tear the tail end of the film itself, given that the hooking action is not applied uniformly across the width of the film but concentrated on the higher side.
Accordingly, the object of the present invention is to overcome the drawbacks mentioned above through the adoption of a solution in which the tail end of a plastic wrapping film is gathered and heat-sealed by a unit featuring compactness in relation to the space afforded by the operating plane, and capable of exerting a precise and uniform grip on the film to bring about the necessary reduction in width.